The present invention relates generally to modem systems that operate with a digital telephone network. More particularly, the present invention relates to signaling techniques for determining the digital characteristics of an end-to-end modem channel.
With the advent of digital modems, e.g., digital pulse code modulation (DPCM) modems compliant with ITU-T Draft Recommendation V.91, end-to-end data transmissions can be performed at theoretical data rates of up to 64 kilobits/second (kbps) where the two modems are digitally connected to a digital telecommunications network. However, because many modem devices are still connected to the telephone network via an analog loop, digital modem devices must remain backward compatible with conventional systems such as V.90 and V.34 modem systems. To enable compatibility with both types of connections (pure digital and an analog/digital combination), an automoding technique is needed to simultaneously determine the connection type while maintaining the integrity of conventional automoding and handshake protocols.
Private branch exchange (PBX) systems and telephone networks outside of the United States may utilize data compression techniques known as adaptive differential pulse code modulation (ADPCM) to reduce the bandwidth of the transmitted signal and to improve the quality of some data transmissions. In contrast to conventional pulse code modulation (PCM) systems that transmit 8-bit codewords at 8 kHz, ADPCM compression techniques enable the data to be transmitted in the form of 4-bit, 3-bit, or 2-bit codewords at 8kHz. Thus, because the codewords are still transmitted at 8 kHz, the effective bit rate is reduced. Conventional modem systems treat ADPCM compression as a form of analog impairment. For example, a conventional V.90 modem system connected over an ADPCM channel will revert to the slower V.34 mode of operation. Unfortunately, the change to V.34 is inherently associated with a slower data rate relative to most V.90 modem connections and relative to an all digital modem connection (as proposed in the draft ITU-T Recommendation V.91).
The initialization and startup routines for a modem system can vary depending upon the characteristics of the current communication channel. For example, the channel may be fully digital, analog, or partially digital. In addition, the channel may be affected by any number of digital (and other) impairments such as robbed bit signaling (RBS), digital pads, encoding law conversions, and any combination thereof. Many data communication systems, such as a V.90 modem system, employ line probing techniques to determine such impairments so that the modem devices can compensate for the impairments. However, these line probing techniques add time to the initialization procedure, which can test the patience of end users who are attempting to establish a fast connection. Furthermore, even though some line probing techniques are unnecessary in the context of certain channel types, conventional modem systems routinely perform line probing procedures in accordance with standard operating protocols.
Accordingly, it would be desirable for a data communication system to implement an efficient and robust inband digital channel determination technique. Although the detection and determination of the digital characteristics of a purely digital end-to-end modem channel may be performed by a devoted signaling or control channel, such a solution may be undesirable because it does not utilize system resources in an optimal manner. In addition, the use of a separate control channel may require a considerable modification to many currently adopted modem standard such as V.90, V.8, and the like.
In accordance with a general embodiment of the present invention, the calling modem transmits digital channel identification (DIDc) sequence to the answer modem near the beginning of the startup routine. The DIDc sequence indicates that the calling modem is digitally connected to the network. The DIDc sequence is also formatted such that the answer modem can determine the characteristics of the communication channel by analyzing the received DIDc sequence. The modem system reacts in an appropriate manner depending upon the detected characteristics of the channel. In this manner, the particular startup procedure can be automatically selected to suit the type of end-to-end channel.
An illustrative embodiment of the present invention is capable of determining whether the current communication channel is an ADPCM channel. If so, the modem system analyzes the specific characteristics of the ADPCM channel and treats the ADPCM compression as if it were a digital impairment. In this manner, the modem system is able to connect at a higher data rate relative to conventional modem systems, e.g., a V.90 system or a system compatible with the Draft Recommendation V.91, that consider ADPCM compression to be an analog impairment associated with an analog channel. The preferred embodiment employs similar techniques to detect and determine other digital impairments such as RBS, digital pads, and encoding law conversions.
The techniques of the present invention may be employed to reduce the startup time associated with conventional modem systems such as V.90 or V.91 systems. Depending upon the detected characteristics of the digital channel, a modem system according to the present invention may be able to eliminate or abbreviate some processes that are mandated by other modem protocols. For example, depending upon the current channel characteristics, the modem system may skip conventional line probing procedures. This feature enables the modem system to reduce the startup latency associated with an end-to-end connection.
The above and other aspects of the present invention may be carried out in one form by a method for initializing a data transmission system having a calling device configured to communicate with an answer device via a digital communication network. The illustrative method involves the establishment of a communication channel between the calling and answering devices, a determination of whether the communication channel is associated with an encoding compression technique, and a determination of the characteristics of the encoding compression technique. The calling device and/or the answering device may then be conditioned to compensate for the characteristics of the encoding compression technique.